In the context of low-carbon society, lower fuel consumption of automobiles is recently strongly requested. With regard to transmissions of automobiles, CVT (continuously variable transmission) (stepless transmission) vehicles accordingly tend to increase.
A CVT is a kind of stepless transmissions made up of two pulleys and one belt. The CVT has a structure in which a pulley portion is made up of a combination of a primary side (input side) and a secondary side (output side) with the belt connecting therebetween. The CVT has a mechanism varying the widths of the pulleys to change the contact radiuses between the pulleys and the belt so that the radius of rotation of the belt varies to change a gear ratio in a stepless manner.
When the gear ratio is changed by varying the widths in the pulley portion, the pulley portion rotates and reciprocates at high speed. A seal used in the pulley portion rotates and reciprocates together. An improvement in sealing performance can therefore contribute to an improvement in fuel consumption. For example, expected effects include elimination of an electric oil pump (reduction in unit weight), application of an idle stop mechanism, and ability to be towed.
An improvement in sealing performance can simply be achieved at low cost by using a rubber squeeze packing. However, since the seal finely vibrates in association with belt rotation, the CVT pulley portion is in an environment in which the rubber squeeze packing is easily worn and, therefore, excellent abrasion resistance is required in addition to the sealing performance.
In general, sealing rings are mainly used as pulley portion seals for CVT. A conventional purpose of seals is to allow pulleys to operate and to retain an oil pressure.
However, the sealing rings are seals for the purpose of retaining a pressure of fluid in a hydraulic circuit rather than the purpose of restraining fluid from leaking outside. Therefore, the sealing rings have a problem of poor sealing properties unable to establish an idling stop system with an electric oil pump eliminated.
On the other hand, D-rings can ensure sealing properties as compared to the sealing rings and, therefore, studies have been conducted for using the D-rings for pulley portion seals for CVT.
CVT pulleys have two sealing portions in a primary portion on the input side and a secondary portion on the output side. As described above, since both the primary portion and the secondary portion are subjected to fine vibrations associated with belt rotation as described above, high friction force is generate at close contact positions between the D-rings and a housing, and the D-rings are required to have abrasion resistance.
The primary portion and the secondary portion have different sealing pressures and the secondary portion is in a higher pressure environment. Therefore, excessive friction generated due to protrusion to an R-portion may damage a protrusion portion in the secondary portion and, as a result, the form of the D-ring may no longer be made. Thus, the D-rings used as pulley portion sealing materials for CVT are required to have pressure resistance.
From above, a material excellent in abrasion resistance and pressure resistance is required.
In this regard, as described in Patent Document 1, a technique has typically been used for containing carbon fibers so as to improve sliding characteristics such as abrasion resistance. In the formulation of Patent Document 1, 65 to 200 parts by weight of carbon fibers are contained per 100 parts by weight of hydrogenated nitrile rubber.
It is also disclosed in Patent Document 1 that a carbon black is contained for improving abrasion resistance characteristics.
It is disclosed also in Patent Documents 2, 3, and 4 that a carbon black may be used in addition to carbon fibers for improving abrasion resistance characteristics.
It is disclosed in Patent Document 5 that, to acquire a highly-elastic rubber composition excellent in crack growth resistance and impact resilience with high heat conductivity, vapor-phase grown carbon fibers and a carbon black are contained in a natural rubber and/or a synthetic rubber. The formulation includes 3 to 60 parts by weight of vapor-phase grown carbon fibers per 100 parts by weight of a natural rubber and/or a synthetic rubber, and a carbon black (with DBP oil absorption of 90 to 140 ml/100 g) having a weight ratio of 0.1 to 13 to the vapor-phase grown carbon fibers (with average diameter of 0.01 to 4 μm). By containing the vapor-phase grown carbon fibers and the carbon black, the concentration of stress of the ends of the vapor-phase grown carbon fibers is alleviated and, therefore, the crack growth resistance inherently possessed by the vapor-phase grown carbon fibers is sufficiently provided.
Patent Document 6 discloses a rubber molded article acquired by crosslinking (vulcanization) of a rubber composition containing a hydrogenation nitrile rubber, a carbon black B and a carbon black C, and/or carbon fibers as a sealing member having appropriate hardness and excellent in formability and abrasion resistance used as a sealing member used in a sealing portion associated with sliding. It is disclosed that the molded article having appropriate hardness and excellent in abrasion resistance is acquired by specifying a carbon black having a DBP oil absorption of 250 to 450 ml/100 g as the carbon black B. It is also described that by using the carbon black (C) having a DBP oil absorption specified to 200 ml/100 g or less, the molded article excellent in abrasion resistance is acquired while maintaining appropriate hardness and favorable formability.